In hydrocarbon recovery operations, stimulation fluids are treatment fluids prepared for stimulation purposes, most commonly matrix stimulation fluids. Most matrix stimulation fluids are acid or solvent-based, with hydrochloric acid being the most common acid due to its reaction characteristics and its relative ease of control. Stimulation generally concerns a treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area.
Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. In the case of acid fracturing acid, the acid etches the formation face creating highly conductive flow paths from the formation back to the wellbore. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art.
Under certain conditions, polymer-gelled fluids will form gas hydrates. A number of hydrocarbons, especially lower-boiling light hydrocarbons, in formation fluids or natural gas or stimulation and other treatment fluids are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in stimulation or other fluid are at least a nuisance for production, handling and transport of these fluids. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and risk of explosion or unintended release of hydrocarbons and other materials into the environment either on-land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions.
There are two broad techniques to overcome or control the hydrocarbon hydrate problems, namely thermodynamic and kinetic. For the thermodynamic approach, there are a number of reported or attempted methods, including water removal, increasing temperature, decreasing pressure, addition of “antifreeze” to the fluid and/or a combination of these. The kinetic approach generally attempts (a) to prevent the smaller hydrocarbon hydrate crystals from agglomerating into larger ones (known in the industry as an anti-agglomerate and abbreviated AA) and/or; (b) to inhibit and/or retard initial hydrocarbon hydrate crystal nucleation; and/or crystal growth (known in the industry as a kinetic hydrate inhibitor and abbreviated KHI). Thermodynamic and kinetic hydrate control methods may be used in conjunction.
Kinetic efforts to control hydrates have included use of different materials as inhibitors. For instance, onium compounds (e.g. ammonium compounds, phosphonium compounds, amine oxides, etc.) with at least four carbon substituents are used to inhibit the plugging of conduits by gas hydrates. Additives such as polymers with lactam rings have also been employed to control clathrate hydrates in fluid systems. These kinetic inhibitors are commonly labeled Low Dosage Hydrate Inhibitors (LDHI) in the art.
Thus, it is desirable if new gas hydrate inhibitors were discovered which would yield comparable or improved results over known gas hydrate inhibitors. It would also be desirable if a composition and method could be devised to inhibit aqueous VES-gelled fluids against the formation of hydrates.